Controlled stripping of aluminide coatings on nickel superalloys through electrolytic techniques

Abstract

Aluminide diffusion coatings are widely employed to improve the oxidation and/or the corrosion resistance of highly added value turbine components operating in harsh environments at high temperatures. Refurbishment of such components requires appropriate removal of worn coatings and of the corrosion products layer-usually an oxide scale. Stripping is mostly carried out using hazardous chemical baths of limited reliability. In this work, an alternative stripping method based on electrochemical techniques has been carried out at laboratory scale for CVD Al diffusion coatings on a directionally solidified Ni base superalloy. Both the galvanostatic and the potentiostatic modes have been investigated. Prior to them, in situ gas bubbling induced by cathodic polarization seems to be an effective way to remove the superficial oxide scales. Measuring the open circuit potential during the experiments allows easy monitoring of the progress of the selective dissolution of the different layers. Complete removal of the aluminide coatings is indicated by potential values similar to those of the substrate. The correlation between the electrochemical features and the surface state after stripping has been carried out by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The electrochemical approach is a promising means to strip out surfaces in a selective and reliable manner.

In-Depth Understanding of the Chemical Stripping Mechanism of AlSiY Coatings on Nickel Superalloys by First-Principles Calculation

“…On the other hand, such a combination of acids should be safe for the surfaces of substrates. Considering the requirements above, many acids and inorganic reagents, such as (NH 4 ) 6 Mo 7 O 24 , HNO 3 , HF and HCl, and their combinations, have been adopted [11][12][13][14]. Poupard et al investigated various soft chemical stripping methods and found that chemical baths containing relatively high concentrations of pitting reagents in organic media are the most effective [15].…”

Comparative study on microstructure evolution and failure mechanisms of ordinary and refurbished EB-PVD TBC under cyclic oxidation

“…TBC removal technique could be roughly divided into two categories: physical removal technologies [16,17], e.g., sandblasting, grinding, polishing, and machining, and chemical removal technologies [18,19], e.g., soft chemical stripping and electrochemical method. TBC is difficult to be completely removed through physical technology due to process characteristics, and strong mechanical impact will exert intense local stress and deformation on metallic components [20,21]. This contradicts the purpose of the refurbished TBC, which is to protect and reuse higher-value metallic components.…”

Development of an imaging system for the detection of alumina on turbine blades

“…Equations (1a) and (1b) show the spontaneous decay of electrons to the ground state from the Ē and 2 Ā states producing the R 1 and R 2 spectral lines characteristic of ruby fluorescence: The repair and overhaul process requires that turbine blades are entirely stripped of their oxidized diffusion coating before a new coating can be deposited [9]. This is achieved by alumina-grit blasting and immersive chemical stripping in hot sulphamic and nitric acid [10]. The effectiveness of the stripping process is currently determined by subjective visual assessments.…”